32 BULLETIN 98, U. S. DEPARTMENT OF AGRICULTURE. 
and if there are any leaks they may be detected by air bubbles rising 
from the joints. 
If the ammonia system has been found to be tight at the pressure of 
150 pounds, close all expansion valves and again start the machine 
and pump air to a gage pressure of about 275 pounds, unless the 
discharge pipe gets very hot. As the low-pressure gage will not 
register so high a pressure as 275 pounds, it should be cut off when 
it has reached the limit of its scale. Allow the system to remain 
under this pressure for several hours and if the loss in pressure 
as shown by the gage, does not exceed 5 pounds, the system may be 
considered satisfactory so far as leakage is concerned. 
Remove the short pieces of pipe between the suction valve and the 
machine and allow the air under pressure to escape quickly through 
•the suction valve. Any dirt and scale that may be in the piping and 
which would otherwise be drawn into the machine with the suction 
gas will be blown out. 
Before the system is charged with the refrigerant it is necessary to 
remove all air and moisture; otherwise the efficient operation of the 
machine will be seriously interfered with. Manufacturers usually 
provide special valves for discharging the air from the system, which is 
accomplished by starting up the machine and pumping the air out, 
the operation being just the reverse of that when working under 
service conditions. When a vacuum of about 26 inches has been 
obtained, stop the machine again and allow it to stand for several 
hours in order to determine if the system will maintain a vacuum. 
If the vacuum is maintained, the system is ready to be charged with 
the ammonia. 
It is impossible in some cases to remove all the air from the system 
by means of the compressor, in which case it is desirable to insert the 
proper amount of refrigerant gradually. Often from 60 to 70 per 
cent of the full charge is inserted, and the air remaining in the system 
is allowed to escape through the purgecocks on the condenser until the 
ammonia shows, which will be detected by the very strong odor, and 
the escaping vapor will have the appearance of steam. An additional 
quantity of ammonia should then be inserted. This should be re- 
peated once or twice a day until all the air has been displaced and the 
complete charge has been introduced. 
CHARGING. 
To charge the machine, a drum of anhydrous ammonia is connected 
by a suitable pipe to the charging valve on the liquid receiver. Allow 
ammonia to enter the system through the charging valve until a 
pressure of about 15 pounds is recorded on the gage and then turn 
on the condensing water and start up the machine slowly at first. 
